1. Field of the Invention
The present invention relates to a pulse-wave characteristic-point determining apparatus which determines a characteristic point of a pulse wave and which is needed, e.g., to calculate a pulse-wave propagation velocity, and also relates to a pulse-wave-propagation-velocity-related-information obtaining apparatus which obtains pulse-wave-propagation-velocity-related information and which employs the pulse-wave-characteristic-point determining apparatus. The pulse-wave-propagation-velocity-related information is information that is related to a velocity at which a pulse wave propagates through an artery between two prescribed portions of a living subject; such as a pulse-wave propagation velocity itself, or a pulse-wave propagation time.
2. Related Art Statement
Obtaining pulse-wave-propagation-velocity-related information involves detecting, from a living subject, two heartbeat-synchronous signals, e.g., a pulse wave, such as a brachial-artery pulse wave, and a heart sound, and determining a time difference between respective times of occurrence of respective characteristic points of respective waveforms of the two heartbeat-synchronous signals. This time difference is the above-mentioned pulse-wave propagation time, and the above-mentioned pulse-wave propagation velocity is obtained by dividing the pulse-wave propagation time by the distance of propagation. Thus, obtaining pulse-wave-propagation-velocity-related information needs determining respective characteristic points of respective waveforms of two heartbeat-synchronous signals. Each characteristic point may be a notch, a rising point, or a maximal point of a heartbeat-synchronous pulse of a pulse wave. The notch is a point where the amplitude of the pulse starts increasing after having decreased following the maximal point.
A time difference between the rising point and the notch corresponds to a time duration in which the left ventricle of the heart contracts, i.e., an ejection period in which blood is ejected from the heart. Thus, the ejection period can be obtained by determining the rising point and the notch as the characteristic points of the pulse wave.
Thus, the pulse-wave-propagation-velocity-related information or the like is obtained based on the times of occurrence of characteristic points of waveforms. Therefore, obtaining accurate pulse-wave-propagation-velocity-related information needs determining accurate times of occurrence of characteristic points.
However, there have been some cases where accuracy of determination of those characteristic points is not sufficiently high. In particular, the degree of clarity of the notch decreases as the distance from the center of the subject increases. For example, for some patients, a pulse wave detected from their upper arm, i.e., a brachial-artery pulse wave shows a considerably clear notch; but it does not for other patients. If the notch is unclear, it is natural that pulse-wave-propagation-velocity-related information or the like obtained based on the notch should be inaccurate. Hence, conventionally, it has been practiced, for accurately determining a time of occurrence of a notch, to pass a signal containing a pulse wave, through a band-pass filter which allows passing of only a signal component having frequencies of the notch, so as to emphasize the notch and determine the location of the notch on the waveform of the pulse wave. However, this method is effective in only those cases where a notch is identifiable to some degree before it is passed through the band-pass filter, and not effective in other cases.